Some of the code is no longer used after changes in the SQL built-in
functions. This patch removes part of the unused code.

Needed for #4145

Part of #4145

Part of #4145

Part of #4145

Part of #4145

This patch refactors UUID() function. Also, UUID(NULL) now returns NULL.

Part of #4145

Part of #4145

Part of #4145

Part of #4145

Part of #4145

This patch refactors RANDOMBLOB() function. Also, RANDOMBLOB(0) now
returns empty string.

part of #4145

Part of #4145

Part of #4145

Part of #4145

Part of #4145

Part of #4145

The CHAR() function now uses the ICU macro to get characters.

Part of #4145

Part of #4145

This patch makes SUBSTR() work according to ANSI rules for SUBSTRING()
function. Also, SUBSTR() can now work correctly with large INTEGER
values. The SUBSTR() syntax has not changed.

Part of #4145

@TarantoolBot document
Title: SUBSTR() function

SUBSTR() now works according to the ANSI rules for SUBSTRING().

Rules for SUBSTR() with 2 arguments:
1) let the first argument be VALUE, and the second argument be START;
2) VALUE should be STRING or VARBINARY, START should be INTEGER;
3) if any of arguments is NULL, NULL is returned;
4) let POS be MAX(START - 1, 0), END be length of the VALUE;
5) if POS >= END, the result is empty string;
6) if POS < END, the result will be substring of VALUE, starting from
   the position POS to the position END.

Rules for SUBSTR() with 3 arguments:
1) let the first argument be VALUE, the second argument be START, and
   the third argument be LENGTH;
2) VALUE should be STRING or VARBINARY, START and LENGTH should be
   INTEGERs;
3) if any of arguments is NULL, NULL is returned;
4) if LENGTH < 0, an error is thrown;
5) let POS be MAX(START - 1, 0), END be START + LENGTH - 1;
6) if POS >= END, the result is empty string;
7) if POS < END, the result will be substring of VALUE, starting from
   the position POS to the position END.

This patch is a refactoring of POSITION(). In addition, VARBINARY
arguments can now be used in this function. In addition, POSITION() now
uses ICU functions instead of self-created.

Part of #4145

This patch refactoring TRIM() and fixes an issue with incorrect trimming
of some VARBINARY values. Also, TRIM() now use ICU functions instead of
self-created.

Part of #4415

Part of #4145

Part of #4145

The CHAR_LENGTH() and CHARACTER_LENGTH() functions now use ICU functions
to determine the length of a string.

Part of #4145

@TarantoolBot document
Title: Invalid UTF-8 values and ICU

Invalid UTF-8 values may be handled differently depending on the ICU
version. For example, for this request:
```
SELECT CHAR_LENGTH(CAST(x'f0808080' AS STRING));
```

On `centos 7` with `libicu-devel-50.2-4.el7_7.x86_64` the result will
be:
```
tarantool> box.execute([[SELECT CHAR_LENGTH(CAST(x'f0808080' AS STRING));]])
---
- metadata:
  - name: COLUMN_1
    type: integer
  rows:
  - [1]
...
```

On `ubuntu 20.04` with `libicu-dev` version `66.1-2ubuntu2` the result
will be:
```
tarantool> box.execute([[SELECT CHAR_LENGTH(CAST(x'f0808080' AS STRING));]])
---
- metadata:
  - name: COLUMN_1
    type: integer
  rows:
  - [4]
...
```

Part of #4145

This patch extends the 'integration.yml' workflow and adds a new
workflow call for running tests to verify integration between tarantool
and the tarantool-php/client connector.

Part of #5265
Part of #6056
Closes #6594

This patch extends the 'integration.yml' workflow and adds a new
workflow call for running tests to verify integration between tarantool
and tarantool-python connector.

Part of #5265
Part of #6056
Closes #6584

Instead of writing to the socket fd directly using sio, we wrap it in
iostream. This will allow us to use complex communication protocols in
iproto.

One thing that should be noted about this patch is how we handle
ev_io_start when we need to wait for the socket to become readable or
writable. Since iostream_write can block because it wants to read from
the socket and iostream_read can block because it wants to write to the
socket, we might need to update input/output events before ev_io_start.
Since ev_io events can't be updated while ev_io is active, we need to
stop ev_io for this.

It's better than using ev_feed_event and ev_is_active directly.

Also, let's use EV_CUSTOM instead EV_READ/EV_WRITE for signaling,
to emphasize that this is an artificial event, which has nothing
to do with fd read/write readiness. It's okay, because input/output
callbacks don't use events at all.

It's better than using sio_socketname directly.

iproto_connection->input.fd != -1 iff state != IPROTO_CONNECTION_ALIVE.
Let's check the state instead of input.fd or output.fd, because it's
easier for understanding.

While we are at it, drop the stale comment to iproto_connection_is_idle:
the function doesn't use evio_has_fd at all.

It's not needed - we can use the iostream created in iproto
(just like a raw fd).

It's not needed - we can use the applier/relay io directly
(just like a raw fd).

Use coio_write_timeout_noxc instead.

Reading/writing fd directly doesn't let us add any data processing
transparently to the users. To overcome this limitation, let's wrap fd
in struct iostream. The new struct exposes virtual read/write methods,
which should be defined by a concrete implementation. It also allows to
access the associated fd, which is needed to poll the stream via libev.
For now, there's the only iostream implementation - plain iostream
without any processing - but we may add other implementations in future.

Apart from introducing the iostream struct, this patch also makes coio
helpers use it. From now on, coio read/write methods take iostream
instead of ev_io and create ev_io internally (in coio_wait). This is
fine, because creation of a new ev_io on stack is cheap.

Basically, this patch updates all coio users so that they call
iostream_create and iostream_close instead of coio_create and
coio_close_io. Plus, it adds a call to iostream_destroy, because in
contrast to ev_io, iostream must be destroyed explicitly.

We are going to wrap fd in iostream struct and pass it to all coio
methods. When we do that, the name 'coio' won't make any sense. Let's
switch to 'io'. We do renaming in a separate patch to reduce the blast
radius of the main patch.